1. Field of the Invention
The present invention relates to a process for producing an inorganic film which can be patterned to produce a patterned inorganic film, and a process for producing a patterned inorganic film. The present invention also relates to an inorganic film which can be produced by using the latter process, and a patterned inorganic film which can be produced by using the former process. The present invention further relates to a piezoelectric device having a piezoelectric film produced by using the former process, and a liquid discharge device using the piezoelectric film.
2. Description of the Related Art
Currently, piezoelectric devices constituted by a piezoelectric film and electrodes are used, for example, as actuators installed in inkjet recording heads, and the like. In the piezoelectric devices, the piezoelectric film expands and contracts in correspondence with increase and decrease in the strength of an electric field which is applied from the electrodes to the piezoelectric film along a predetermined direction. The perovskite oxides such as PZT (lead titanate zirconate) are known as materials suitable for the piezoelectric film.
It is known that in uncontinuous piezoelectric films each of which is patterned so as to form a plurality of separate protruding portions, the separate protruding portions can smoothly expand and contract, and therefore great displacement can be achieved. In order to achieve desired displacement, the piezoelectric films are formed, for example, with thicknesses of 1 to 5 micrometers. The thicknesses of the piezoelectric films are greater than the thicknesses of the electrodes, which are on the order of nanometers (e.g., 200 nm). As disclosed in U.S. Pat. No. 6,436,838. Conventionally, the patterning of piezoelectric films of PZT or the like is generally performed by dry etching.
The dry etching is generally known as anisotropic etching. However, PZT and the like are uneasy to etch, and the piezoelectric films are thicker than the electrodes (the thicknesses of which are on the order of nanometers). Therefore, the piezoelectric films are more difficult to etch than the electrodes and the like, so that there is a tendency that patterning of the piezoelectric films takes much time. In addition, the dry etching cannot achieve completely anisotropic processing, so that the protruding portions have tapered shapes. Thus, there is a tendency that highly precise etching is difficult.
In order to improve the quality of images produced by the inkjet recording heads, demands for higher uniformness in the piezoelectric performance of the protruding portions constituting a piezoelectric film used in each inkjet recording head are increasing. However, it is difficult to equalize the taper angles of the side faces of the protruding portions by the dry etching, which produces tapered shapes of the protruding portions. Therefore, the influence, on the image quality, of variations in the piezoelectric performance which are caused by variations in the shapes of the protruding portions can become unignorable in the future. In consideration of the precision in the shapes of the side faces of the protruding portions, it is preferable to use a patterning technique which can stably produce approximately vertical side faces of the protruding portions.
In addition, the lift-off technique is known as a technique for patterning an inorganic film such as an electrode and a dielectric film, as indicated in Japanese Unexamined Patent Publication No. 53 (1978)-070764, U.S. Pat. No. 6,156,672, and U.S. Patent Application Publication No. 20040248047. According to the lift-off technique, removable resist is selectively deposited on areas of a substrate on which an inorganic film is not to be formed in accordance with a predetermined pattern. Then, a solid (gapless) inorganic film is formed over the substrate on which the above removable resist is selectively formed. Finally, the resist is removed together with the portions of the inorganic film located on the resist, so that the other portions of the inorganic film in the predetermined pattern are left.
However, since the inorganic film is formed on the resist according to the lift-off technique, it is impossible to form the inorganic film above the heat-resistance temperature. The inorganic films are reported to be formed at 150° C. or lower by Japanese Unexamined Patent Publication No. 53 (1978)-070764 and U.S. Pat. No. 6,156,672, and at 250° C. or lower by U.S. Patent Application Publication No. 20040248047. In order to achieve satisfactory growth of a perovskite structure in a piezoelectric film of a perovskite oxide such as PZT, the film-formation temperature of 350° C. or higher (preferably, 400° C. or higher) is necessary. Therefore, it is impossible to use the lift-off technique (which uses resist) for formation of the piezoelectric film.
The lift-off technique can realize precise patterning of a thin film such as an electrode having a thickness on the order of nanometers and formation of approximately vertical side faces of protruding portions. However, when the lift-off technique is used in patterning of a piezoelectric film, which has a great thickness on the order of micrometers, the great thickness of the piezoelectric film located on the resist makes the removal of the resist and the unnecessary portions of the piezoelectric film difficult. In addition, since the unnecessary portions of the piezoelectric film, which is originally formed in a continuous single piece, are torn from the necessary portions of the piezoelectric film for the removal, it is difficult to form the pattern with satisfactorily precise shapes.